1. Field of the Invention
The present invention relates to a gas sensor for measuring gas components such as NO, NO.sub.2, SO.sub.2, CO.sub.2, and H.sub.2 O contained in, for example, atmospheric air and exhaust gas discharged from vehicles or automobiles. The present invention also relates to a method for controlling the gas sensor.
2. Description of the Related Art
Recently, a gas sensor 10A as shown in FIG. 8 has been known, which is based on the use of an oxygen ion conductor (for example, see Japanese Laid-Open Patent Publication No. 8-271476).
The gas sensor 10A is operated as follows. That is, a measurement gas existing in the external space is introduced into a first hollow space 14 via a first diffusion rate-determining means 12. A first oxygen pumping means 22, which comprises an inner pumping electrode 16, an oxygen ion conductor 18, and an outer pumping electrode 20, is used to pump in or pump out oxygen contained in the measurement gas in the first hollow space 14 to such a degree that the nitrogen oxide as a measurement objective is not decomposed.
Subsequently, the measurement gas in the first hollow space 14 is introduced into a second hollow space 26 via a second diffusion rate-determining means 24. A second oxygen pumping means 36, which is disposed for the second hollow space 26 and which comprises a measurement gas-decomposing electrode 28, an oxygen ion conductor 30, and a reference electrode 34 disposed in a reference air section 32, is used to pump out oxygen produced by decomposition effected by the catalytic action of the measurement gas-decomposing electrode 28 or the electrolysis caused by voltage application. A current value, which is required to pump out oxygen by using the second oxygen pumping means 36, is measured to indirectly measure the nitrogen oxide.
Examples of practical use of the gas sensor 10A include, for example, NOx sensors, H.sub.2 O sensors, and CO.sub.2 sensors for measuring measurement gases containing those having bound oxygen.
When the conventional gas sensor 10A is applied as an NOx sensor, for example, Rh or Pt is used for the measurement gas-decomposing electrode 28 to catalytically decompose NOx. The oxygen, which is produced during the decomposition, is detected as a pumping current, or the oxygen is detected as a change in voltage.
When the conventional gas sensor 10A is applied as an H.sub.2 O sensor or a CO.sub.2 sensor, it is difficult to perform catalytic decomposition. Therefore, a voltage, at which each of the gases is decomposable, is applied to the second oxygen pumping means 36. The oxygen, which is produced by electrolysis caused by the voltage application, is detected as a pumping current.
By the way, in the case of the conventional gas sensor 10A described above, a GND line of a DC power source 38 for controlling the first oxygen pumping means 22 cannot be used in common with that of a DC power source 40 for controlling the second oxygen pumping means 36, because of the following reason. That is, the leak current flows from the outer pumping electrode 20 to the measurement gas-decomposing electrode 28, or the leak current flows from the measurement gas-decomposing electrode 28 to the inner pumping electrode 16.
When the current flows through the oxygen ion conductor, the movement of oxygen occurs, in accordance with which the control operation may become unstable, and the pumping current for measurement may be affected. Consequently, it is feared that the measurement cannot be performed.
Therefore, the conventional gas sensor 10A requires two DC power sources which are insulated from each other, for driving the first and second oxygen pumping means 22, 36.
On the other hand, a gas sensor 10B shown in FIG. 9 has been suggested. The gas sensor 10B includes an auxiliary pumping electrode 42 provided in the second hollow space 26 to construct a third oxygen pumping means (i.e., auxiliary pumping means) 46 by the auxiliary pumping electrode 42, oxygen ion conductors (18, 44, 30), and the reference electrode 34. Accordingly, the oxygen, which diffuses to cause invasion in an minute amount from the first hollow space 14, is pumped out again to greatly improve the measurement accuracy (especially, the dependency on oxygen concentration) (Japanese Laid-Open Patent Publication No. 9-113484).
The illustrative suggested gas sensor 10B requires as much as three DC power sources which are insulated and independent from each other, due to the addition of the auxiliary pumping means 46. FIG. 10 shows a control circuit system of the illustrative suggested gas sensor 10B shown in FIG. 9. In this case, the three DC power sources (first DC power source 50A, second DC power source 50B, and third DC power source 50C), which are insulated and independent from each other, are used to control the first, second, and third oxygen pumping means 22, 36, 46.
The first DC power source 50A is used as a power source for a pumping control circuit 52 for controlling the first oxygen pumping means 22. In the pumping control circuit 52, an electromotive force between a measuring electrode 54 and the reference electrode 34 is detected by a first comparator 56. Subsequently, a difference with respect to a target voltage (for example, 300 mV) is determined by a second comparator 58, and the differential voltage is amplified by an amplifier 60. The amplified voltage is applied, as a control voltage E.sub.0, between the outer pumping electrode 20 and the inner pumping electrode 16 of the first oxygen pumping means 22. Thus, the first oxygen pumping means 22 is controlled.
The second DC power source 50B is used as a power source for supplying a voltage E.sub.1 to the auxiliary pumping means (third oxygen pumping means) 46. Specifically, a constant voltage is obtained by using a Zener diode 62. After that, a voltage E.sub.1 to be applied to the auxiliary pumping means 46 is generated by using a voltage-dividing circuit 64, which is applied to the auxiliary pumping means 46.
The third DC power source 50C is used as a power source for supplying a voltage E.sub.2 to the second oxygen pumping means 36. The voltage E.sub.2 to be supplied to the second oxygen pumping means 36 is generated in accordance with a method similar to that used in the second DC power source 50B, which is supplied to the second oxygen pumping means 36.
As shown in FIG. 11, each of the mutually insulated and independent three DC power sources (hereinafter referred to as "insulated type power source", while a power source, which does not require the insulated and independent arrangement, is hereinafter referred to as "non-insulated type power source") 50A, 50B, 50B is basically constructed by an oscillation circuit 72, an insulated type transformer 74, and a rectifier circuit 76 connected to downstream positions of a battery 70 (for example, 12 V in the case of a car battery). In such an arrangement, the non-insulated type power source can be constructed by using only semiconductor parts such as transistors and operational amplifiers, while the insulated type power source as described above requires the transformer 74. Therefore, it is difficult for the insulated type power source to miniaturize the control circuit system of the gas sensor 10A, 10B and reduce the weight thereof, and an inconvenience is feared in that the production cost becomes expensive.